1 Matrix Lie Groups
1.1 Definition of a Matrix Lie Group
We begin with a very important class of groups, the general linear groups. The groups we will study in this book will all be subgroups (of a certain sort) of one of the general linear groups. This chapter makes use of various standard results from linear algebra that are summarized in Appendix B. This chapter also assumes basic facts and definitions from the theory of abstract groups; the necessary information is provided in Appendix A. Definition 1.1. The general linear group over the real numbers, denoted GL(n; R), is the group of all n × n invertible matrices with real entries. The general linear group over the complex numbers, denoted GL(n; C), is the group of all n × n invertible matrices with complex entries. The general linear groups are indeed groups under the operation of matrix multiplication: The product of two invertible matrices is invertible, the iden- tity matrix is an identity for the group, an invertible matrix has (by definition) an inverse, and matrix multiplication is associative.
Definition 1.2. Let Mn(C) denote the space of all n×n matrices with complex entries.
Definition 1.3. Let Am be a sequence of complex matrices in Mn(C). We say that Am converges to a matrix A if each entry of Am converges (as →∞ m ) to the corresponding entry of A (i.e., if (Am)kl converges to Akl for all 1 ≤ k, l ≤ n). Definition 1.4. A matrix Lie group is any subgroup G of GL(n; C) with the following property: If Am is any sequence of matrices in G, and Am converges to some matrix A then either A ∈ G,orA is not invertible. The condition on G amounts to saying that G is a closed subset of GL(n; C). (This does not necessarily mean that G is closed in Mn(C).) Thus, Definition 4 1 Matrix Lie Groups
1.4 is equivalent to saying that a matrix Lie group is a closed subgroup of GL(n; C). The condition that G be a closed subgroup, as opposed to merely a sub- group, should be regarded as a technicality, in that most of the interesting subgroups of GL(n; C) have this property. (Most of the matrix Lie groups G we will consider have the stronger property that if Am is any sequence of matrices in G,andAm converges to some matrix A,thenA ∈ G (i.e., that G is closed in Mn(C)).)
1.1.1 Counterexamples
An example of a subgroup of GL(n; C) which is not closed (and hence is not a matrix Lie group) is the set of all n×n invertible matrices all of whose entries are real and rational. This is in fact a subgroup of GL(n; C), but not a closed subgroup. That is, one can (easily) have a sequence of invertible matrices with rational entries converging to an invertible matrix with some irrational entries. (In fact, every real invertible matrix is the limit of some sequence of invertible matrices with rational entries.) Another example of a group of matrices which is not a matrix Lie group is the following subgroup of GL(2; C). Let a be an irrational real number and let eit 0 G = t ∈ R . 0 eita Clearly, G is a subgroup of GL(2, C). Because a is irrational, the matrix −I is not in G,sincetomakeeit equal to −1, we must take t to be an odd integer multiple of π,inwhichcaseta cannot be an odd integer multiple of π.Onthe other hand (Exercise 1), by taking t =(2n +1)π for a suitably chosen integer n,wecanmaketa arbitrarily close to an odd integer multiple of π. Hence, we can find a sequence of matrices in G which converges to −I,andsoG is not a matrix Lie group. See Exercise 1 and Exercise 18 for more information.
1.2 Examples of Matrix Lie Groups
Mastering the subject of Lie groups involves not only learning the general the- ory but also familiarizing oneself with examples. In this section, we introduce some of the most important examples of (matrix) Lie groups.
1.2.1 The general linear groups GL(n; R) and GL(n; C)
The general linear groups (over R or C) are themselves matrix Lie groups. Of course, GL(n; C) is a subgroup of itself. Furthermore, if Am is a sequence of matrices in GL(n; C)andAm converges to A, then by the definition of GL(n; C), either A is in GL(n; C), or A is not invertible. 1.2 Examples of Matrix Lie Groups 5
Moreover, GL(n; R) is a subgroup of GL(n; C), and if Am ∈ GL(n; R)and Am converges to A, then the entries of A are real. Thus, either A is not invertible or A ∈ GL(n; R).
1.2.2 The special linear groups SL(n; R) and SL(n; C)
The special linear group (over R or C) is the group of n × n invertible matrices (with real or complex entries) having determinant one. Both of these are subgroups of GL(n; C). Furthermore, if An is a sequence of matrices with determinant one and An converges to A,thenA also has determinant one, because the determinant is a continuous function. Thus, SL(n; R)andSL (n; C) are matrix Lie groups.
1.2.3 The orthogonal and special orthogonal groups, O(n) and SO(n)
An n × n real matrix A is said to be orthogonal if the column vectors that make up A are orthonormal, that is, if